This invention relates generally to the joining of metals, and, more particularly, to diffusion bonding of non-ferrous metals at elevated temperatures and pressures.
In many applications of metals, a metal piece must be joined to another metal piece to create a useful structure. It has long been a common practice to join metal pieces with conventional fasteners such as bolts, rivets, or screws, or with specialized fasteners. Metal pieces are also joined by welding or brazing, using a filler metal between the joined pieces.
These conventional techniques are usually successful in joining the pieces, but may not produce the strongest and most durable joint possible because of stress concentrations, irregularities, and incomplete bonding at the interface between the pieces joined. The stress concentrations, irregularities and incomplete bonds often act as the points of initiation of cracks and thence failure during service, so that the joint becomes the primary source of weakness in the structure.
As a solution to the joining problem for structures that are to be used at room temperature or low elevated temperatures, specialized organic adhesives such as epoxies have been developed. The pieces to be joined are essentially glued together by the adhesive. The adhesive spreads the loads transmitted through the joint across the entire area of the joint, reducing the incidence of failure due to stress concentrations. However, the strength of such adhesives falls rapidly with increasing temperature, so that pieces to be used at temperatures greater than about 250.degree. C. cannot be joined with such adhesives.
Some metals may be joined together by cleaning a smooth surface on each and subsequently pressing them together under pressure, in a nonreactive environment and at elevated temperature. This process is known as diffusion bonding. The material in the two pieces interdiffuse slightly, and the grains of the two pieces grow across the interface, so that in reality the interface disappears. When two pieces of the same or similar composition and microstructure are diffusion bonded together properly, it is often impossible to discern where the interface was, even with high power microscopes. Moreover, the joint becomes as strong and durable as the underlying pieces being bonded, and is not a favored site for failure initiation. This diffusion bonding process is therefore highly preferred where it can be used to advantage.
Not all metals can be readily diffusion bonded For example, a tough oxide scale at the surface, as found on aluminum alloys, prevents interdiffusion. The oxide can sometimes be broken by mechanical working and/or chemical cleaning, but a remnant typically remains in the interfacial region as a source of defects
In some cases, nature has provided metals having a desirable combination of properties useful in particular applications, coupled with the ability to be diffusion bonded. One commercially more important of such metals is titanium and its alloys. Titanium dissolves its own oxide at elevated temperatures, removing this impediment to the diffusion bonding process.
A sufficient pressing pressure must be applied for a period of time in order to provide enough plastic flow or deformation to remove irregularities and voids at the interface, and force the two pieces into full contact along the entire interface. The titanium alloys are often selected because of their high strength at elevated service temperatures, but this high strength acts to slow the diffusion bonding operation at a selected temperature by reducing the rate of plastic flow necessary to achieve full bonding. The rate of flow can be increased by increasing the bonding pressure, but this approach reduces the size of pieces that can be bonded by use of a mechanical bonding press of a particular force capacity Alternatively, if the bonding pressure if provided by isostatic pressing, more complex and expensive equipment is required to achieve higher pressures.
Normally, effective diffusion bonding temperatures for titanium alloys are relatively high, to increase the rate of diffusion so that voids can be eliminated and bonding achieved without excessive pressure or excessive bonding time. For conventional alpha-beta titanium alloys such as Ti-6Al-4V, diffusion bonding temperatures are usually selected which range from about the beta transus temperature (about 995.degree. C.) to well below the beta transus temperature (about 870.degree. C.)
These high bonding temperatures are often undesirable, since they can cause phase coarsening and poor mechanical properties in the finished article. High bonding temperatures are also undesirable when reinforcing particles or fibers must be included within a diffusion bonded article as in fabrication of metal-matrix composites. In this case, high bonding temperatures can cause reaction between the included particles or fibers and the matrix metal, severely reducing the strength of the finished composite article Lowering of diffusion bonding temperatures is an important goal in further improving diffusion bonding operations,
There have been several approaches to enhancing diffusion bonding :f titanium alloys. In one, the alloy composition is changed to reduce elevated temperature flow strength. Bonding is thereby enhanced, but the ultimate usefulness of the bonded structure is reduced In another approach, bonding is accelerated by coating the surfaces to be bonded with a fugitive coating that accelerates bonding, but then diffuses away into the bulk of the metal during the bonding operation. This method introduces undesired impurities into the final structure, particularly near to the bond line.
In still another approach described in U.S. Pat. No. 3,713,207, a specially prepared thin, fine-grained, superplastic foil interlayer is placed between the pieces to be bonded. The superplastic properties of the foil reduce bonding pressures and/or temperatures However, preparation of the foil material is often difficult or prohibitively expensive, making this approach undesirable for many diffusion bonding needs.
Accordingly, there exists a continuing need for an improved technique for bonding pieces of titanium and other types of alloys that can inherently be diffusion bonded. Such a technique should provide for reduced bonding temperatures and/or pressures, and should not adversely affect the performance of the final bonded structure, as by leaving an incompletely bonded joint, residue, or impurities. The present invention fulfills this need, and further provides related advantages.